Report Suggests Link Between Smallmouth Bass Mortality and Water Pollution

Smallmouth bass fish kills occurred in Monocacy River in Western Maryland, other rivers around Chesapeake Bay watershed

(ANNAPOLIS,MD)—Scientists suggest that a perfect storm of pollution, parasites, warming water temperatures, and endocrine disrupting chemicals may be combining to threaten one of the region's most popular fresh water game fish, the smallmouth bass, according to a new Chesapeake Bay Foundation (CBF) report. In Maryland recreational anglers targeting smallmouth bass contribute $206 million to local economies, supporting 1,000 jobs, according to data from the American Sportfishing Association and the U.S. Fish and Wildlife Service.

"Smallmouth bass are one of the most prized game fish," said CBF President Will Baker. "But in the Monocacy River a fish kill in 2009 wiped out about 65 percent of the smallmouth bass population. The good news is that implementing Maryland's Clean Water Blueprint can make a difference."

An investigation of that 2009 kill found that 29 percent of the fish in the Monocacy had lesions on their skin. Recent sampling in the Monocacy River found that the smallmouth bass population has yet to fully recover, according to the Maryland Department of Natural Resources.

Catch rates of smallmouth bass in the middle Susquehanna River declined by more than 80 percent between 2001 and 2005, and they have not rebounded since, according to the Pennsylvania Fish and Boat Commission.

In Maryland's Monocacy River, data from the U.S. Geological Survey indicates high levels of both nitrogen and phosphorus pollution between 2007 and 2011.

Scientists believe that nitrogen and phosphorus pollution may be contributing to the fish deaths and diseases in two ways. The first is by spurring the growth of parasites (myxozoans and trematoads) and their hosts (worms and snails). The second is by feeding algal blooms that raise pH levels and lower oxygen concentrations, stressing young smallmouth bass.

Between 82 and 100 percent of male smallmouth bass in the Potomac and Shenandoah rivers have sexual abnormalities. Scientists have also found intersex fish in the Susquehanna River, where between 89 and 93 percent of the male fish sampled had sexual abnormalities. The cause remains unknown, however a recent study found 135 different chemicals in the water over smallmouth bass nesting sites.

Smallmouth bass do not tolerate pollution well. In fact, some suggest that smallmouth bass may be like a canary in the coal mine indicating future possible health problems for other species of fish. High levels of nitrogen and phosphorus pollution, rising water temperatures, and endocrine disrupting chemicals may all have combined to weaken the immune systems of smallmouth bass and make them more susceptible to naturally occurring bacteria, viruses, and parasites.

Maryland's Clean Water Blueprint lays out the steps that need to be taken to reduce pollution and improve water quality in local rivers, streams, and the Chesapeake Bay. Urban and suburban stormwater is the only pollution source that is continuing to increase, damaging fish and crabs, and putting human health at risk. CBF encourages the state and local jurisdictions to aggressively implement the plans that have been developed and put in place necessary funding sources.

John Mullican, a fisheries manager at the Maryland Department of Natural Resources, said that there are no clear population trends up or down for smallmouth bass in the Potomac River, although reproduction has been down over the last five years.

"I don't see an immediate catastrophic issue for the population," Mullican said. "But we are concerned about the mortalities in the Monocacy River, as well as the effects endocrine disruption may have on smallmouth bass reproduction."